Conventionally, the dimensions of liquid crystal display ("LCD") cell gaps are controlled by spacers or spacer posts. Cell gap uniformity is determined by the size, density and distribution of the spacers. Spacers ensure that the LCD panels have correct cell gaps and support substrates, thereby preventing them from collapsing onto each other. However, liquid crystal molecules adjacent to the spacers in the display area are distorted from the corresponding bulk orientations. As a result, there are light leakages at the spacer locations. The problem is more pronounced in projection displays where miniature displays are viewed at ten or hundred times magnification. There, spacers appear as defective spots, degrading contrast ratio and image quality.
One approach to addressing this problem has been to place the spacers out of the display or viewing area, i.e. in the peripheral area and in between the pixel gap. For direct view displays, spacers can be hidden within the inter-pixel gap which, measuring about 10 .mu.m, is larger than the cell gap. Therefore, the whole panel can be supported evenly. For projection displays, however, the inter-pixel gap is only about 1 .mu.m or less. In this case, the spacers are visible even when placed at the corner of the pixel. Therefore, the spacers must be limited to the peripheral area in projection display devices to eliminate the spacer visibility problem.
FIG. 1 is an illustration of a conventional prior art LCD cell assembly method. Typically, during LCD panel assembly, an external force or pressure is applied over the entirety of one or both of the substrates by means of mechanical, compressed air or vacuum to press the substrates so that they both contact the spacers. In FIG. 1, external force is applied to a rigid plate 100 and a flat platform 110 which are disposed against the outside surfaces of two substrates 120 and 130, respectively. As a result of the pressure, the substrates 120, 130 both are made to contact the spacers 140. After the pressure is released, the panels either maintain a rather uniform cell gap or bowl up due to built-in stress. In the case where the panels bowl up, the cell gap will be corrected by another press during the end seal process after the injection of liquid crystal.
If, in this arrangement, the spacers 140 were eliminated from the viewing area to improve viewing quality, there would be no support to withstand the external pressure. For displays which have smaller than a half inch diagonal, the rigidity of the substrates 120, 130 might be enough to hold the gap without collapsing. However, for displays larger than a half inch, the two substrates will collapse onto each other. As a result, the cell gap would not be uniform and viewing quality would diminish.
One attempt to address this problem is disclosed in U.S. Pat. No. 5,499,127 issued Mar. 12, 1996 to Tsubota et al. In Tsubota et al., the invention of which is shown in FIG. 2, spacers 240 are located in a curing resin that forms the sealing member of the LCD devices. The two substrates 220, 230 (one coated with the sealing member) are initially pressed together with pressing plates 200, 210 until the spacers 240 are against each substrate, thereby forming a panel of the devices (not shown). Next, an elastic sheet 250 of glass fiber is disposed on the top substrate 220 adjacent the pressing plate 200. Portions of this elastic sheet 250 is cut out so that the remaining sheet is positioned over and in between the various sealing members 240. Upon a second pressing, the top substrate 220 is deformed such that the gap between the substrates 220, 230 is narrow at and outside the sealing member and is wide inside the sealing member. Thereafter, the sealing member is heat cured to reduce the built-in stress of the glue and a liquid crystal injection apparatus is used to restore some cell gap uniformity.
The method disclosed in Tsubota et al. has the following problems. First, the use of the single elastic sheet creates stress on the LCD device such that LCD cells are produced with 3-5 fringes associated with the Newton rings which are caused by the interference of the light from the two inner surfaces of the liquid crystal cell cavity when the cell gap is not uniform. In other words, cell gap distortion occurs and uniformity suffers. Second, the stress created by the disclosed method cannot be completely eliminated even after aneals before and after subsequent liquid crystal injection and end sealing. To improve the cell gap uniformity, the reduction of built-in stress is essential. Sealant breakage is very possible under this stress level. Third, since the substrates are not 100% rigid and flat, the bottom substrate 230 disposed directly against the pressing plate 210 is made to conform to the surface of that plate and away from that of the upper substrate 220. This results in a non-uniform cell gap. Finally, particles trapped between the bottom substrate 230 and the adjacently disposed plate 210 will distort the display.
It is an object of the present invention to provide an apparatus and method for assembling LCD devices which do not suffer from the problems listed hereinabove.